The use of plastic bottles composed of moldable plastics, such as polyethylene terephthalate (PET), has greatly expanded during the last few decades. Because such plastic bottles are almost unbreakable, weigh only about one-tenth the weight of glass, have excellent clarity and do not impart any taste to their contents, such bottles have become ubiquitous in today's society. As with all containers to be filled aseptically, plastic bottles must be functionally sterilized to remove trace contaminants such as bacteria (e.g. C. botulinum) or molds prior to being filled in order to attain aseptic filling and storage longevity requirements. Generally, containers are pre-produced and stored in bulk quantities until they are ready to be used and therefore are prone to contamination under normal storage conditions. Some plastic containers are stored as plastic tubes or plugs, and are blow molded with hot air prior to entering the filling machine; because of the low melt temperature of the plastic bottle, the inner surface is not necessarily sterilized during this blow molding step and requires additional treatment. Plastic bottles offer a unique challenge in that they cannot withstand the harsher treatments (time and temperature) that can be afforded to glass or metal containers in order to achieve sterility prior to filling. Accordingly, before these bottles can be filled with consumables, it is essential that they be sterilized in order to ensure that the risk of contamination by pathogenic microorganisms is minimized. Because of the vast number of such plastic bottles required to satisfy consumer demand, it is desirable that such sterilization be accomplished rapidly without any sacrifice in efficacy.
While peracetic acid (which is also called peroxyacetic acid) “PAA” is a known sterilizing agent, the prior art indicates that aqueous formulations of PAA are not suitable for the rapid sterilization of containers made out of PET. Thus, U.S. Pat. No. 6,790,380 discloses that in order to prevent the formation of harmful bacteria it is necessary to raise the temperature or concentration of a sterilizing agent, or to prolong a treating time. However, such publication indicates that none of these options are desirable for the sterilization of PET using aqueous PAA—heating the PAA solution tends to deform PET bottles; while raising the concentration results in undesirably high residues of hydrogen peroxide and/or acetic acid. Prolonging the treatment time is not desirable as this will slow down the process considerably.
U.S. Pat. Nos. 6,536,188 and 6,945,013 disclose the use of hydrogen peroxide fogs to disinfect the interior of PET bottles. These publications further indicate that oxonia (a mixture comprising 15-40 weight percent hydrogen peroxide; 7-13 weight percent acetic acid; and 5-10 weight percent PAA) may also be employed. However, these publications further indicate that it is necessary to first activate such sterilant and then to remove it using a plurality of drying stations.
United States Patent Application 2010/0196197 discloses the use of a true vapor (as opposed to a fog which contains suspended liquid particles) of a diluted PAA solution to sterilize surfaces. However, such publication indicates that such vapor should be employed at a temperature of between about 80 and about 120 degrees C., and for a contact time of between about 15 and 40 minutes. It is noted that the glass transition temperature of PET is about 75 degrees C., and that therefore PET bottles may deform at higher temperatures. Further, such an extended contact time is not amenable to the rapid sterilization of a large numbers of bottles.
Accordingly, it would be desirable if a method of rapidly sterilizing PET bottles employing PAA as the sterilant could be developed.